Challenges
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PLC programming challenges
Challenge 06 · Find the fault · Free

The output that won’t listen

Start does nothing. Jog works. Find out why.

Find why Start does nothing and fix it. Start latches the pump on and Stop stops it. Holding Jog runs the pump for maintenance, only while held. Stop wins over Start and Jog.

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The 3D pump skid this challenge runs on
Test replay

Operator report: Start runs the pump

Test 1 of 4
Set by the testChecked by the testStep 3 of 3 · 300 ms
Level
Easy · 10 points
Kind
Find the fault
Time
About 8 min
Pass rate
New
Machine
3D pump skid
Siemens
LAD and SCL
Rockwell
Ladder and Structured Text
Beckhoff
Structured Text
The operator’s report · pump skid
P-101 won’t run in Auto. I press Start and nothing happens. It only runs while someone holds the Jog button. Nobody has touched the start and stop logic. The Jog rung was added last month.
Automatic checks

What the checks test

  1. Start latches PumpOn ON; it stays on after Start is let go.
  2. Jog runs PumpOn only while it is held.
  3. Stop turns PumpOn OFF and wins over Start and Jog.
  4. Jogging a running pump leaves it running.

The checks run 4 tests on the machine, scan by scan: Operator report: Start runs the pump; Jog runs it only while held; Stop wins; Jogging a running pump leaves it running.

On site

Where you’d find this

A pump skid where a maintenance jog button was added to the program months after it was first written.

A double coil hides in plain sight: each rung looks right on its own. It is one of the first things an experienced engineer checks when an output ignores its logic.

The machine

How it should behave

Start
Latches the pump on.
Jog
Runs it only while held.
Stop
Stops it, and wins over Start and Jog.

Press Run, then Start. The pump should start and keep running. Hold Jog with the pump stopped: it runs only while held.

Keep going

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